People are compelled to search for renewable clean fuel as petrochemical fuel is in shortage more than ever and pollutes the environment upon combustion. In a substitutive solution for petrochemical fuel, producing clean fuel from renewable biological oils and fats (such as soybean oil, Jatropha oil, lard, food waste oil) is considered as a green, renewable, carbon-neutral technical route.
Biological oils and fats, such as vegetable oils and fats, which are fats obtained by extracting seeds, the flesh of fruit, and other parts of plants, contain a lot of triglycerides with long-chain carbons and free fatty acids. According to their usage, biological oils and fats can be divided into two categories—to be used for food and to be used in industry. Those in liquid at room temperature are called oils, while those in solid and semi-solid at room temperature are called fats. The components of biological oils and fats are quite different from those of crude oil (including waste lubricating oil) mainly in that, for biological oils and fats, the content of oxygen is high and the contents of sulfur, nitrogen and aromatic hydrocarbons are low; while for crude oil, contents of sulfur, nitrogen and aromatic hydrocarbons are high and content of oxygen is low. Therefore, due to the differences of their components, methods for producing fuel from crude oil as raw material are not applicable to the processing of biological oils and fats as raw materials.
Currently, there are some technical solutions, which are used or will be used in industry, for preparing biomass fuel (such as biodiesel) from biological oils and fats. Generally, such technical solutions are classified into two categories: processes of transesterification and processes of direct hydrogenation. Many research results and technical reports involve the foregoing techniques.
Chinese Patent Application Publication No. CN 101328418A teaches a method for manufacturing biodiesel by using vegetable oils. However, a great amount of ethanol is consumed firstly to form fatty acid ester through reacting with the vegetable oil. Chinese Patent Application Publication No. CN1858161A teaches a method for preparing biodiesel from palm oil, in which, it is required that the raw material of palm oil proceed with the procedures, such as degumming, deacidifying and dehydrating, etc. before the step of esterifying with low-carbon alcohols. Chinese Patent Application Publication No. CN101230309A teaches a method for preparing biodiesel through reducing the high acid value of palm oil, in which, two esterification procedures are required. Moreover, methanol required in the method is in an amount 6-9 times (molar ratio) of that of the oil in the trans esterification reaction. Furthermore, methanol is not recycled in the process, and therefore the method is neither environmentally friendly or economical.
The aforementioned processes in the prior art are complicated in regard to the procedures, are difficult to operate and require high energy consumption. Moreover, a high amount of alcohols, such as methanol or ethanol etc., is involved in the process of transesterification, which results in the production cost increasing greatly.
Besides, Chinese Patent Application Publication No. CN101070483A teaches a method for producing biodiesel with Saueda Salsa seed oil, in which a lot of water is required for washing the product after transesterification. Chinese Patent Application Publication No. CN1412278A teaches a method for preparing biodiesel by utilizing high acid oils and fats as well as palm oil together with a strong acid as catalyst. However, when the method proceeds, a lot of waste water would be produced and the reactor would corrode badly. All aforementioned processes in the prior art produce a lot of waste water, which not only increases production costs, but also goes against environmental protection and economic benefit.
Regarding direct hydrogenating technique, WO2009/039347 teaches a process for producing diesel fractions by treating biorenewable feedstock, in which a two-step-approach, i.e., hydrodeoxygenation and hydroisomerization, is employed. US2006/0207166 has a process, in which hydrodeoxygenation and hydroisomerization take place simultaneously. A common defect of these processes lay in the poor stability of the catalyst and the high consumption for hydrogen, which will be more serious for vegetable oils and animal fats with high oxygen contents.
Particularly, the technique of direct hydrogenation is restricted by the contents of free fatty acids in raw materials. Up to now, only raw material containing up to 15% of free fatty acids is disclosed in the prior art documents, which is used to produce hydrocarbon fuel through direct hydrogenation (Yanyong Liu et al., Chem. Lett. 2009, 38, 552).
In general, in the processes currently used for preparing diesel fuel from biological oils and fats, transesterification is employed to produce biodiesel by means of transesterification from biological oils and fats in which a lot of low-carbon alcohol is consumed and therefore, production costs are increased. In the transesterification process, a strong acid is employed as a catalyst which corrodes the production apparatuses badly, and at the same time, a follow-up separation procedure is required for handling a large amount of glycerol, which is a by-product generated in the process. After the transesterification process, a large volume of waste water will be produced. In some transesterification processes, multiple transesterification procedures are involved, which results in the operation being very cumbersome. On the other hand, direct hydrogenation produces diesel through hydrodeoxygenation for animal and vegetable oils and fats directly, which is not only of high hydrogen consumption, but also of fast catalyst inactivation. Since the oxygen content in raw material oils is 10%-15%, a large amount of reaction heat will be produced. It is a hard-resolved problem to control the reaction temperature so as to prevent catalyst from fast inactivation. Furthermore, a large amount of hydrogen gas for replenishing and for quenching is required to maintain the hydrogen partial pressure as the consumption of hydrogen gas for processing the raw material oils is relatively high.
There are still some other processes in the art for preparing diesel fuel by using biological oils and fats. For example, US2006/0186020 discloses a process of coprocessing vegetable oils and crude oil, in which the content of vegetable oils is between 1-75% and vegetable oils are not used alone. Chinese Patent Application Publication No. CN10101314748A discloses a method for catalytically converting animal and vegetable oils and fats to obtain a product with mainly C2-C4 alkenes as its ingredients and only 45% overall yield. Only a small amount of components for gasoline and diesel can be produced from the process. Furthermore, there is no hydrorefining procedure for gasoline and diesel involved in the process.
Chinese Patent Publication No. CN101475870A teaches a process of catalytic cracking distillation for hydrocarbons consisting of mainly waste lubricating oil sources. In this process, the waste lubricating oil to be treated mainly consists of alkanes, and the catalytic cracking breaks down C—C bonds selectively. The main reaction is shown as follows:R1—CH2—CH2—R2→R1—CH3+CH2═R2 
No water (H2O) is produced in this cracking reaction, thus the water-resistance of the catalyst is not required to be taken into account. Waste lubricating oil mainly consists of alkanes, which form alkanes and alkenes directly after catalytic cracking. However, as above mentioned, the composition of biological oils and fats is quite different from that of crude oil (including waste lubricating oil), in which the main components of biological oils and fats have high oxygen content. If cracking is carried out, factors, such as C—O bond cracking and water produced, should be taken into account, as there are great differences in catalytic mechanism and the hydrothermal stability of the catalyst. As a result, the process taught by Chinese Patent Application Publication No. CN101475870A is not applicable to biological oils and fats.
In conclusion, as for the prior art, although there are many routes and research results disclosed for processing and manufacturing biodiesel by using biological oils and fats as raw materials, biodiesel prepared by these processes is not considered by far an ideal diesel blending component as the obtained biodiesel has high density, low blending ratio with diesel components from petroleum, low heat value, and is less economical as a fuel when it is blended with diesel components from petroleum.
Contents of the Invention
In order to solve the above problems, the present invention provides a novel process for treating biological oils and fats. High quality biomass fuel that is very suitable to be used as a blending component of diesel is obtained.
Specifically, the present invention provides a method for preparing biomass fuel, in which biological oils and fats are used as raw materials and the composition of the biomass fuel thus obtained is equivalent to that obtained from crude oil refining. The process comprises the following steps: (a) proceeding with a catalytic cracking-deoxygenation reaction under heating in the presence of a cracking-deoxygenation catalyst for the biological oils and fats; (b) mixing the product of step (a) with hydrogen gas; and (c) proceeding with a catalytic hydrodeoxygenation reaction under heating in the presence of a hydrodeoxygenation catalyst for the mixture from step (b). According to actual demands, the product of step (c) may be further fractionated. Generally, it may be also considered to mix hydrogen gas with the product of catalytic cracking-deoxygenation reaction before the hydrogen gas is injected into the reaction reactor for the reaction of step (c), whereas the hydrogen gas may also be injected into the reaction reactor directly followed by being mixed with the product of catalytic cracking-deoxygenation reaction for the reaction of step (c).
As for the process of the present invention, it is characterized in that a double-deoxygenation-procedure, i.e., a catalytic cracking-deoxygenation procedure and a catalytic hydrodeoxygenation procedure, is employed, and therefore, the defects of large heat release and fast inactivation of the catalyst caused by the direct hydrogenation used in the prior art can be avoided, and the consumption of hydrogen gas can be reduced greatly at the same time. Moreover, each procedure in the process of the present application, e.g., the catalytic cracking-deoxygenation procedure and the catalytic hydrodeoxygenation procedure, can be combined flexibly, i.e., can operate either continuously or separately. Waste residue and waste gas produced from the process can be utilized comprehensively for heating, which makes the whole production process more energy efficient and environmentally friendly.
Reference signs for main assemblies are as follows:    1 Distillation still    2 Catalytic distillation tower    3 Condenser    4 Vapor-liquid separator    5 Liquid feeding pump    6 Heating furnace    7 Hydrorefining reaction reactor    8 Hydrogen gas positive booster    9 Atmospheric distillation tower